1. Field of the Invention
The present invention relates to a mobile communication system and, in particular, to a preamble transmission method for a wireless communication system that is capable of increasing the probability of successful transmission.
2. Description of the Related Art
Typically, Wideband Code Division Multiple Access (WCDMA) systems, also known as Universal Mobile Telephone Systems (UMTS), utilize the Random Access Channel (RACH) and the Common Packet Channel (CPCH) as uplink transport channels.
FIG. 1 is a diagram illustrating a session initiation procedure in a conventional wireless communication system.
Referring to FIG. 1, in order for a user equipment 110 to place a call to a particular telephone number, the user equipment 110 sends a request for a connection to a base station 130 by means of an RACH preamble (S100). The RACH is an uplink transport channel employed by the user equipment 110 to connect to the base station 130. When an RACH preamble is received from the user equipment 110, the base station 130 sends an acknowledgement (ACK) message to the user equipment 110 through an Acquisition Indicator Channel (AICH) (S103). After receiving the Ack message, the user equipment 110 and base station 130 initiate an exchange of channel establishment messages (S105).
The RACH preamble is characterized by the use of open loop power control unlike other dedicated channels. As such, the user equipment 110 calculates an initial transmission power on the basis of a current channel environment and transmits a preamble with the initial transmission power. The RACH preamble initial transmission power is calculated using equation 1.Preamble_Initial_Power=Primary_CPICH—Tx_Power−CPICH_RSCP+UL_Interference+Constant_Value  Equation 1wherein Primary CPICH_Tx_Power is a transmission power of a Common Pilot Channel (CPICH), CPICH_RSCP is a received signal code power strength at the user equipment, and the UL_Interference is an uplink interference amount.
In equation 1, all values except for the CPICH_RSCP are obtained from a system information message transmitted by the base station 130. The CPICH_RSCP is measured at the user equipment 110.
The system information message is broadcast over the entire cell, i.e., the coverage area of the base station 130. The user equipment in the coverage area of the base station attempts connections to the base station. In other words, the user equipment 110 receives the system information broadcast by the base station 130 and transmits the RACH preamble on the basis of the received system information. The RACH-related information includes Primary_CPICH Tx_Power, UL_Interference, Constant_Value, a number of maximum preamble ramping cycle (Mmax), a maximum preamble retransmission cycle (Rmax), preamble power increment (ΔP0), ith preamble transmission power (Pi), and transmission power offset (Poffset).
After transmitting the preamble with the initial power calculated using equation 1, the user equipment 110 awaits the Ack which is transmitted by the base station 130 through the AICH. If the Ack is not received, the user equipment 110 retransmits the preamble with the transmission power increased by power ramp step (powerRampStep) until reaching the maximum number of preamble retransmission times (preambleRetransMax). The transmission power of nth preamble is calculated by equation 2.P(n)=Preamble_initial_Power+powerRampStep*(n−1) 0≦n≦preambleRetransMax)  Equation 2
If the preamble retransmission number reaches the preambleRetransMax, the user equipment 110 measures the CPICH_RSCP again and repeatedly retransmits the preamble on the basis of the measured CPICH_RSCP as many times as Mmax.
If no Ack is received even with the continuous retransmission, the user equipment 110 tries to transmit the preamble as many times as preableRetransMax×Mmax.
In a case of radio resource control (RRC) connection request, the preamble retransmission may occur as many times as N300. In this case, the user equipment 110 retransmits the preamble totally preambleRetransMax×Mmax×N300 times. The RRC is a sub-layer responsible for efficient radio resource assignment.
Such a preamble unreachable problem can be caused by inefficient cell planning and inaccurate open loop power control in the time-varying radio frequency environment.
In such manner, the transmission power increases whenever the preamble is retransmitted at the ramping cycle (powerRampStep).
However, this conventional power control mechanism has a drawback in that the power consumption increases whenever the preamble is retransmitted. Furthermore, in a case that the initial transmission power (Preamble_Initial_Power) is determined at a low level, the preamble transmission failure probability increases even with the stepwise power increment.
That is, the conventional preamble transmission method is disadvantageous in terms of power consumption and RACH procedure failure caused by frequent retransmission, resulting in degradation of service reliability.